Research

The Center on Interfacial Engineering in Microelectromechanical Systems (CIEMS) is advancing the surface-science and engineering of microstructural materials, coatings, and processes to enhance the capabilities and performance of micro and nanoelectromechanical systems, through funding interdisciplinary, collaborative research projects at Stanford University, the University of California at Berkeley, and Iowa State University.

Motivation

A well-known outcome of scaling into the micro and nanometer dimensions is the dominance of surface over bulk phenomena in determining electronic, mechanical, and electromechanical behavior. The existing knowledge base is on surface phenomena is concentrated on narrow material combinations in specific packaging environments. The incomplete knowledge about MEMS surfaces and their role in determining fracture and fatigue mechanisms has resulted in designers adopting “rules of thumb” which ensure large safety margins. Such conservative designs result in lower sensitivities for devices that depend on the absolute amplitude of motion, such as vibratory rate gyroscopes, and limit the range of microactuators in applications such as micro-optical scanners. The expansion of MEMS into new applications, especially those involving contact or exposure to harsh environments, has been hamstrung by the lack of basic understanding of structural surfaces, surface coatings, and microencapsulation processes and how they impact the performance of MEMS.

Microshell Test Environment

Central to the research strategy of CIEMS is to utilize recent progress in sealed microcavity encapsulation to establish a benchmark, reproducible microenvironment for characterizing surface and interface properties. Prof. T. Kenny and his group at Stanford, together with Bosch RTC have recently demonstrated such a sealed microcavity process. After patterning the structural layer, an additional layer of SiO₂ sacrificial material is deposited and then the first layer of the polysilicon encapsulation. After patterning etch-access holes, the microstructure is released by vapor-HF etching and then fully sealed by deposition of conformal LTO or polysilicon. The microcavity process has several advantageous features for creating a standard microenvironment for evaluating surface properties. The device is sealed in an ultrapure environment at high temperature with hydrogen as a residual gas and never exposed to external environments after the encapsulation is complete. After sealing, wafers or individual devices can be heated to 500C or higher, while maintaining the ultrapure environment.

Surface Micro/Nano Instrumentation

The use of the encapsulated microshell environment as the benchmark for characterizing surface-related phenomena requires new MEMS test structures that can be actuated and interrogated inside the microshell. Actuators must be designed that are compatible with the design rules for the sealed cavity; in addition removal of the microshell without damaging the underlying device will be necessary to resolve some surface phenomenon. Fatigue and especially fracture test structures will prove more challenging to implement inside the microshell, due to the high-force actuators required. Since the microshell maintains its integrity at high temperatures, accelerated life testing procedures for key MEMS failure modes will be investigated.

Ultra-Low Mechanical Loss Surfaces

Microshell-encapsulated resonators have already demonstrated sub-ppm stability of both resonant frequency and quality factor over hundreds of hours of operation and repetitive thermal cycling. To enhance further the performance of resonators and sensors based on resonant-frequency shifts, it is essential to understand the respective roles of surface passivation, bond termination, and roughness. The microshell encapsulation process, combined with robust surface coatings, provides flexibility in probing these phenomena.

Robust Coatings for Harsh Environments

The characterization of many harsh-environment coatings has been limited to ambient conditions, a narrow range of contacts, and without any external fields, leaving unanswered many questions about coating quality factor and fatigue. CIEMS will work to develop new, stable coatings for achieving low-energy surfaces for microshell and harsh-environment applications. Research toward new, robust MEMS coatings is aided by the insights from informatics tools under development in K. Rajan’s group at Iowa State. By using a “rational drug design” approach to targeting potential new crystal chemistries, “virtual” compound chemistries with targeted modulus properties can be developed. This approach will be extended to cover a broad range of refractory metal alloys, oxides and nitrides and oxy-nitrides and will be complemented by ab initio calculations to study interactions with these materials.

Molecular Vapor Deposited Organic Surface Functionalization

Much of the academic research in self-assembled monolayer (SAM) coating has been based on formation in liquids, which has several drawbacks for commercial application. The new Molecular Vapor Deposition (MVD^TM) technique enables the design of entirely new classes of low-energy, wear-resistant organic coatings, which can be deposited on released micro or nano structures. Given the near-room temperature deposition temperatures, this tool could be used to functionalize the surface of polymer-based microfluidic systems. CIEMS researchers will explore new post-release low-temperature coating, motivated by molecular models of surface interactions.

Photonic Meta Materials

The major barrier to deployment of optical MEMS is the demanding requirements for its packaging, which must combine optical access and alignment with hermetic sealing of critical components. Building on the fundamental advances in interfaces and coatings, CIEMS will investigate a new approach to wafer-level, encapsulated optical MEMS based on the creation of photonic meta-materials. These materials are patterned on sub-wavelength length scales, to achieve the high reflectivity and filter characteristics that traditionally can only be achieved with metals and dielectric stacks. Unlike traditional metal-coated optical MEMS, such meta-materials can withstand the microshell encapsulation process.